Has the Earth's sixth mass extinction
already arrived?
2 March 2011
principal author Anthony D. Barnosky, UC Berkeley
professor of integrative biology, a curator in the
Museum of Paleontology and a research
paleontologist in the Museum of Vertebrate
Zoology.
"If currently threatened species - those officially
classed as critically endangered, endangered and
vulnerable - actually went extinct, and that rate of
extinction continued, the sixth mass extinction
could arrive within as little as 3 to 22 centuries," he
said.
Nevertheless, Barnosky added, it's not too late to
Tigers are one of Earth's most critically endangered
save these critically endangered mammals and
species. Extinction of the majority of such species would
other such species and stop short of the tipping
indicate the sixth mass extinction is in our near future.
point. That would require dealing with a perfect
Credit: Anthony Barnosky, UC Berkeley
storm of threats, including habitat fragmentation,
invasive species, disease and global warming,
With the steep decline in populations of many
animal species, from frogs and fish to tigers, some
scientists have warned that Earth is on the brink of
a mass extinction like those that occurred only five
times before during the past 540 million years.
Each of these 'Big Five' saw three-quarters or
more of all animal species go extinct.
"So far, only 1 to 2 percent of all species have gone
extinct in the groups we can look at clearly, so by
those numbers, it looks like we are not far down the
road to extinction. We still have a lot of Earth's biota
to save," Barnosky said. "It's very important to
devote resources and legislation toward species
conservation if we don't want to be the species
whose activity caused a mass extinction."
In a study to be published in the March 3 issue of
the journal Nature, University of California,
Berkeley, paleobiologists assess where mammals
and other species stand today in terms of possible
extinction, compared with the past 540 million
years, and they find cause for hope as well as
alarm.
Coauthor Charles Marshall, UC Berkeley professor
of integrative biology and director of the campus's
Museum of Paleontology, emphasized that the
small number of recorded extinctions to date does
not mean we are not in a crisis.
"If you look only at the critically endangered
mammals - those where the risk of extinction is at
least 50 percent within three of their generations and assume that their time will run out, and they
will be extinct in 1,000 years, that puts us clearly
outside any range of normal, and tells us that we
are moving into the mass extinction realm," said
"Just because the magnitude is low compared to
the biggest mass extinctions we've seen in a half a
billion years doesn't mean to say that they aren't
significant," he said. "Even though the magnitude is
fairly low, present rates are higher than during most
past mass extinctions."
"The modern global mass extinction is a largely
unaddressed hazard of climate change and human
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activities," said H. Richard Lane, program director in To get around this limitation, Marshall said, "This
the National Science Foundation's Division of Earth paper, instead of calculating a single death rate,
Sciences, which funded the research. "Its continued estimates the range of plausible rates for the mass
progression, as this paper shows, could result in
extinctions from the fossil record and then
unforeseen - and irreversible - negative
compares these rates to where we are now."
consequences to the environment and to
humanity."
Barnosky's team chose mammals as a starting
point because they are well studied today and are
well represented in the fossil record going back
some 65 million years. Biologists estimate that
within the past 500 years, at least 80 mammal
species have gone extinct out of a starting total of
5,570 species.
The team's estimate for the average extinction rate
for mammals is less than two extinctions every
million years, far lower than the current extinction
rate for mammals.
Earth's warming climate is contributing to an infection
responsible for tropical frog extinctions. Credit: Nicolle
Rager Fuller, National Science Foundation
"It looks like modern extinction rates resemble
mass extinction rates, even after setting a high bar
for defining 'mass extinction,'" Barnosky said.
After looking at the list of threatened species
maintained by the International Union for
Conservation of Nature (IUCN), the team
The study originated in a graduate seminar
concluded that if all mammals now listed as
Barnosky organized in 2009 to bring biologists and
"critically endangered," "endangered" and
paleontologists together in an attempt to compare
"threatened" go extinct, whether that takes several
the extinction rate seen in the fossil record with
hundred years or 1,000 years, Earth will be in a
today's extinction record. These are "like comparing
true mass extinction.
apples and oranges," Barnosky said. For one thing,
the fossil record goes back 3.5 billion years, while
"Obviously there are caveats," Barnosky said.
the historical record goes back only a few thousand
"What we know is based on observations from just
years. In addition, the fossil record has many holes,
a very few twigs plucked from the enormous
making it is impossible to count every species that
number of branches that make up the tree of life."
evolved and subsequently disappeared, which
probably amounts to 99 percent of all species that
He urges similar studies of groups other than
have ever existed. A different set of data problems
mammals in order to confirm the findings, as well
complicates counting modern extinctions.
as action to combat the loss of animal and plant
species.
Dating of the fossil record also is not very precise,
Marshall said.
"Our findings highlight how essential it is to save
critically endangered, endangered and vulnerable
"If we find a mass extinction, we have great
species," Barnosky added. "With them, Earth's
difficulty determining whether it was a bad weekend
biodiversity remains in pretty good shape compared
or it occurred over a decade or 10,000 years," he
to the long-term biodiversity baseline. If most of
said. "But without the fossil record, we really have
them die, even if their disappearance is stretched
no scale to measure the significance of the impact
out over the next 1,000 years, the sixth mass
we are having."
extinction will have arrived."
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Provided by University of California - Berkeley
APA citation: Has the Earth's sixth mass extinction already arrived? (2011, March 2) retrieved 18 June
2017 from https://phys.org/news/2011-03-earth-sixth-mass-extinction.html
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